Cellulose food casings are well known in the art and are widely used in the production of stuffed food products such as sausages and the like. Cellulose food casings generally are seamless tubes formed of a regenerated cellulose and contain a plasticizer such as water and/or a polyol such as glycerin. Plasticization is necessary because otherwise the cellulose tube is too brittle for handling and commercial use.
Cellulose food casings generally are used in one of two forms. In one form the casing consists of a tubular film of pure regenerated cellulose having a wall thickness ranging from about 0.025 mm to about 0.076 mm and made in tube diameters of about 14.5 mm to 203.2 mm. The second form is a reinforced casing wherein the tubular wall of the casing consists of a regenerated cellulose bonded to a paper web. Such reinforced casings are commonly called "fibrous" casings to distinguish them from the nonreinforced cellulose casings. Fibrous casings have a wall in the range of 0.050 mm to 0.102 mm thick and are made in diameters of about 40.6 mm to 193 mm or greater.
The cellulose for making both types of casings is most commonly produced by the so-called "viscose process", wherein viscose, a soluble cellulose derivative, is extruded as a tubular film through an annular die into coagulating and regenerating baths to produce a tube of regenerated cellulose. This tube is subsequently washed, plasticized with glycerin or other polyol, and dried. Drying usually is accomplished while the tube is inflated with air at a pressure sufficient both to maintain a constant tube diameter and to orient the film.
The viscose process for making cellulose is well known in the art. Briefly, in the viscose process a natural cellulose such as, wood pulp or cotton linters, is treated with a caustic solution to activate the cellulose to permit derivatization and extract certain alkali soluble fractions from the natural cellulose. The resulting alkali cellulose is shredded, aged and treated with carbon disulfide to form cellulose xanthate which is a cellulose derivative. The cellulose xanthate is dissolved in a weak caustic solution. The resulting solution or "viscose" is ripened, filtered, deaerated and extruded. The pulp source and time of aging the alkali cellulose are selected depending upon whether the viscose will be used to make fibrous casing or nonreinforced cellulose casing. Fibrous casing uses a less viscous solution because the lower viscosity solution wicks into the paper web, completely penetrating the web, establishing strong intercellulose bonding. A more viscous solution is used to extrude a nonreinforced cellulose casing.
The viscose is extruded as a tube through an annular die and about a self-centering mandrel into coagulation and regenerating baths containing salts and sulfuric acid. In the acidic baths the cellulose xanthate, e.g., viscose, is converted back to cellulose. In this respect the acid bath decomposes the cellulose xanthate with the result that a pure form of cellulose is coagulated and regenerated. Initially, the coagulated and regenerated cellulose is in a gel state. In this gel state the cellulose tube first is run through a series of rinse water dip tanks to remove by-products formed during regeneration. The gel tube then is treated with a glycerin humectant and dried to about 10% moisture, based on total casing weight. The gel tube is inflated during the drying process to a pressure sufficient to provide a degree of orientation to the dried cellulose tube.
Both nonreinforced cellulose casings and fibrous casings are produced in this fashion except that in the case of fibrous casings the viscose is extruded onto a tube of paper prior to entering the coagulation and regenerating baths.
During regeneration of the cellulose from the xanthate solution, sulfur products are liberated and gases such as hydrogen sulfide, carbon disulfide and carbon dioxide are released through both the inner and outer surfaces of the gel tube. The gases produced as by-products during regeneration are noxious and toxic, so their containment and recovery imposes a considerable burden on the manufacturing process. Moreover, gases generated at the internal surface of the extruded tube can accumulate within the tubular casing and consequently present special problems. The tubular casing, while in its gel state, is expansible and the pressure build up of gases accumulating within the gel casing causes undesirable diameter variations. To prevent this, the gel casing is punctured periodically to vent the accumulated gases. This puncturing process, involving procedures to puncture, vent, and then seal the punctured gel tube, results in an undesirable interruption of the manufacturing process. Also, gases which evolve within the casing wall may become entrapped causing bubbles which weaken the casing and detract from its stuffability.
The casing in its gel state to some extent retains low residual levels of the sulfur compounds produced during regeneration. While care is taken to remove all residual sulfur compounds by washing the gel tube prior to drying, the dried casing may still contain trace amounts of these compounds.
Despite the problems inherent with the viscose process as described above, nevertheless it is the most commonly used process for the production of cellulose casing for the food processing industry.
An alternate cellulose production method involves forming a cellulose solution by means of a simple dissolution rather than requiring prior derivatization to form a soluble substance. A cellulose dissolution process is described in U.S. Pat. No. 2,179,181. This patent discloses the dissolution of natural cellulose by a tertiary amine oxide to produce solutions of relatively low solids content, for example, 7 to 10% by weight cellulose dissolved in 93 to 90% by weight of the tertiary amine. The cellulose in the resulting solution is nonderivatized prior to dissolution. U.S. Pat. No. 3,447,939 discloses use of N-methyl-morpholine-N-oxide (NMMO) as the cyclic amine solvent where the resulting solutions, while having a low solids content, can be used in chemical reactions involving the dissolved compound or to precipitate the cellulose to form a film or filament.
More recent patents such as U.S. Pat. No. 4,145,532 and U.S. Pat. No. 4,426,288 improve upon the teachings of the '939 patent. U.S. Pat. No. 4,145,532 discloses a process for making a solution of cellulose in a tertiary amine oxide such as NMMO that contains 10-35% by weight of cellulose. This higher solids content, achieved in part by including an amount of water (from 1.4% to about 29% by weight) in the tertiary amine oxide solvent, provides a solution adapted for shaping into a cellulosic article such as by extrusion or spinning. In U.S. Pat. No. 4,426,288, the NMMO-cellulose solution contains an additive which reduces decomposition of the cellulose polymer chain so that molding or spinning substances are obtained with only slight discoloration and that will yield molded shapes distinguished by improved strengths upon precipitation in a nonsolvent such as water.
Cellulose dissolution generally occurs by four methods: it behaves as an electron donor base, or an electron acceptor acid, or complexes with another reagent, or forms a derivative in which the cellulose is covalently bonded through alcohol groups with various reagents to form new molecules. The latter includes sodium cellulose xanthate, ie., cellulose being an alcohol can react to make esters such as a xanthate derivative that is soluble in aqueous, nonaqueous or strongly polar organic solvents. The solubilization is mainly due to the disrupting of the hydrogen bonding by the derivative bonds. The salient feature of this step is that the derivatizing groups can be easily removed by hydroxylic materials such as, for example, aqueous acid, yielding pure cellulose. When dissolved by a derivatization process, the cellulose is truly regenerated, whereas when dissolved by complexation or solvation of pure cellulose, as in the first three mentioned types of dissolution, the cellulose is mainly precipitated or coagulated, i.e., reorganized into a shape. Notwithstanding these differences, the resulting cellulose article is chemically identical irrespective of whether it is reprecipitated from solutions or chemically regenerated.
Using NMMO as a solvent for cellulose eliminates the need for derivatizing the cellulose as in the viscose process. Consequently, it eliminates the disadvantages attendant to the viscose process, such as, the problems associated with the generation of toxic and noxious gases and sulfur compounds.
However, while nonderivatized cellulose resulting from the process of dissolving cellulose in NMMO eliminates certain problems associated with the viscose process, to applicant's knowledge, NMMO-cellulose solutions have not yet been used in the manufacture of cellulose food casings. It is speculated that nonderivatized cellulose has not been commercially used in manufacture of food casings because the solution at 65.degree. C. has a viscosity significantly higher than the viscosity of derivatized cellulose used in the production of cellulose food casings. In particular, nonderivatized cellulose in solution may have a molecular weight of about 80,000 to 150,000 and a viscosity in the range of about 1,000,000 to 3,500,000 centipoise. The high molecular weight and viscosity results because the dissolution of the cellulose does not affect the degree of polymerization. Viscose for casing manufacture, where the degree of polymerization is affected by the derivatization process, has a molecular weight in the range of about 95,000 to 115,000 for nonfibrous casing and a viscosity of 5,000 to 30,000 centipoise.
From a cellulose article manufacturing process standpoint, these differences are important because after dissolution the process steps, including cellulose recovery, are dependent on whether cellulose has entered into a covalent bond with the solubilizing reagent, i.e., has been derivatized. This is so in the case of the well-known and commercially practiced viscose process. When a cellulose derivative is processed into the shaped article, the derivative, such as viscose, is first partially coagulated in the extrusion bath and then subsequently hydrolyzed back to cellulose, i.e., cellulose is regenerated. During this hydrolysis and while the derivative is still in a "plastic" state, the reforming cellulose crystallites can be stretched and oriented to give desirable-commercial properties such as high tensile strength or burst strength. However, a disadvantage of this general approach is that since a cellulose derivative has been hydrolyzed, additional byproducts are formed. This significantly complicates cellulose recovery.
By contrast, in the nonderivative cellulose dissolution methods such as NMMO/H.sub.2 O, orienting the cellulose molecules during the reorganization of the cellulose article is more difficult because there is no covalent bond to break. So, reorganization is essentially a physical dilution or decomplexation. However, recovery is less complex and, at least in the cellulose/NMMO/H.sub.2 O system, commercially feasible.
U.S. Pat. No. 4,246,221 and East German Patent No. DD 218 121 have taught that such nonderivatized cellulose containing mixtures with NMMO and water may be forced through a nozzle and longitudinally guided through a 12 inch long air gap into a precipitating bath to form very small diameter solid fibers. More recently, the nonderivatized cellulose fiber spinning literature teaches that such long air path lengths should be avoided. For example, U.S. Pat. No. 5,252,284 ("'284"), states that a long air gap leads to sticking of the fibers, uncertainties in spinning and fiber breakage at high degrees of drawing. According to '284, by using selected orifice diameters and nozzle channel lengths, the air gap is desirably reduced to at most 35 mm (1.4 inches).
However, the manufacture of individual solid cellulose fibers by extrusion through orifices of 2-4 mils diameter is nonanalogous to manufacture of cellulose food casings which are extruded as a hollow tube of at least about 700 mils inside diameter with wall thickness typically on the order of 40 mils.
Food casings made of derivatized cellulose typically contain additives or coatings to enhance food processing and food characteristics. For example, colorants are incorporated in or on the casing to make self-coloring casings, which transfer the color during processing of the food product from the casing to the food product. Liquid smokes, which impart a smoky flavor and a reddish color to the food product, are also incorporated in or coated on the casing. Peeling aids that allow the casing to be completely stripped off the cooked meat product without causing any of the meat product to be damaged, are also added to the casings.
Casings are made in a variety of colors. Dye pigments are incorporated into the cellulose prior to extrusion to produce these colored casings. At times, printing is needed on the outside of the casing. Certain dyes and pigments can be used to print whatever is needed on the casing.
While the ability of a derivatized cellulose casing to accept these additives and coatings has been demonstrated, there has been no experience nor expectation that a nonderivatized cellulose casing would likewise accept these coatings and additives. Accordingly, it is a principle object of the present invention to provide a cellulosic food casing made from nonderivatized cellulose that incorporates a coating or additive that enhances the performance of the food casing and a method for preparing the same. Typical additives or coatings desirable for incorporation into the casing especially are those that transfer colors or flavors to the food product from the casing, those that assist in the peeling of the casing from the meat, and the dyes that are used to print on the surface of the casing.